What is Minimum Control speed VMC, or VMCA?

VMC or VMCA?

VMCA, the Minimum Control speed
in the Air (or Airborne), is one of the Minimum Control speeds (VMC's)
of a multi-engine airplane that is published as operational limitation in its
Airplane Flight Manual (AFM). Other
published VMC's are Minimum Control speed on the Ground (VMCG)
and, in some cases, also the Minimum Control speed during approach and Landing (VMCL).
VMC is often used in manuals rather than VMCA. Regulations,
however, are changing VMC into VMCA, because "VMCA
is more commonly used" (Flight Test Guide
AC 25-7C).

Minimum Control Speed - VMCA

When an engine fails or is inoperative
in-flight, the rudder is used to counteract the asymmetrical thrust yawing
moment. Roll effects are counteracted by the ailerons. The
counteracting forces generated by the aerodynamic control surfaces are
proportional to the square of the airspeed (V2), to the area of
the control surfaces (S) and to the air density. For a given size of
the vertical tail with rudder, there is a speed below which the generated
rudder side force is not large enough to counteract the asymmetrical thrust,
or below which the ailerons are not effective anymore: the heading and/or
bank angle cannot be maintained below this speed. This speed is called
Minimum Control speed VMC or better: Minimum Control speed in the
Air VMCA.

VMCA is already determined (i.e.
assumed) by the design engineer for sizing the vertical tail (fin). A
vertical tail may not be that small that VMCA increases above 1.2
VS
(FAR 23.149). On the other side, a large tail results in a smaller VMCA
but in higher weight and production cost.
FAR 23 allows the design engineer to use a small bank angle of maximum 5º
(away from the failed engine) which can be used to reduce the size of the
vertical tail and also reduce both the sideslip (drag) and VMCA
while an engine is inoperative. A larger bank angle increases the
sideslip and might result in a fin stall.
However, the saved hardware weight of a smaller tail needs to be replaced by
a quite 'heavy' software condition (on paper in the AFM) for pilots when an
engine is inoperative. This condition is presented below.

Refer to the paper Control and Performance
during Asymmetrical Powered Flight on the downloads page (#2)
for details on tail design and the use of VMCA.

The course on
asymmetric powered flight that test pilots and flight test engineers receive
at formal Test Pilot Schools can be downloaded from the References list (No.
10 and 11),
here.

Bank angle versus sideslip and VMCA during straight flight
for a sample airplane when engine #1 is inoperative. This graph shows:When Indicated Airspeed is near VMCA, then bank 4° away from the
inoperative engine to avoid loss of control and for minimum drag, i.e. for
max. Rate of Climb.

This graph shows (actual) VMCA and sideslip angle versus bank angle of
a sample airplane after failure of the left engine (#1).
The airspeed that results from the bank angle for which the sideslip is zero
is the VMCA that will be published in the AFM (85 kt). At bank
angles larger than 6° away from the failed engine, for this sample airplane,
the sideslip angle increases to 14°, the maximum possible angle of attack of
the fin. The airspeed needs to be increased to avoid the fin to stall -
actual VMCA
increases (blue line).
Notice that the actual VMCA for wings level (100 kt) of this
sample airplane is 15 kt higher than the AFM published VMCA.
Therefore the pilot should not maintain a bank
angle of maximum 5°
(either side) as is presented in VMC definitions in most AFM's,
but
the exact bank angle that was used to design the vertical tail and at
which the drag is minimal (in this example 4°, usually 5°).

For Manufacturers:

For Pilots:

FAR and EASA/CS 23.149 and equivalent
present the definition of VMCA for the design and
certification of multi-engine airplanes that is also
inappropriately copied into most AFM's:

VMC
is the calibrated airspeed at which, when the critical engine is suddenly
made inoperative, it is possible to maintain control of the airplane with
that engine still inoperative, and thereafter maintain straight flight at
the same speed with an angle of bank of not more than 5 degrees.

Once the airplane is designed and build, the selected tail size imposes
a limitation on, i.e. a constraint to, pilots. The VMCA
definition for use by pilots is therefore different than the VMCA
definition out of FAR/CS 23.149 that is for manufacturers, for designing
and certification of airplanes.

VMCA
is the minimum speed for maintaining straight flight when an engine
fails or is inoperative and the corresponding opposite engine is set to
provide maximum thrust, provided a bank angle is being maintained of 3 –
5 degrees (exact number to be provided by the manufacturer) away from
the inoperative engine.

In addition, the manufacturer should specify the configuration for which
this, or other published VMCA's is/ are valid.

For further details, refer to the papers for pilots presented on the
Downloads page.

On the airspeed indicator of Part 23 twin-engine airplanes, the
standardized AFM-published VMCA is indicated by a red radial
line, in this example at 80 kt. However, neither a placard on the
instrument panel nor a note or warning in the AFM tells the pilot that the
redlined VMCA is valid only if a bank angle of 3 to 5 degrees (to be
specified by the manufacturer) is maintained away from the inoperative
engine. Any other bank angle results in a much higher actual
VMCA and to the loss of control after which an accident
cannot be avoided (when asym. thrust is not reduced).

The airspeed for maximum single-engine rate of climb VYSE
is indicated by a blue radial line, here at 105 kt. In the
legend of some Performance Data Tables of Graphs, a note tells the pilot
that the presented performance data, including the performance at VYSE,
are valid only if a small bank angle is being maintained of 2 - 3 degrees
away from the inoperative engine. For other bank angles, the maximum climb
performance or the performance to maintain altitude (i.e. to prevent
drifting down) is not guaranteed.

If the pointer is at or near the red line and the thrust on the
remaining engine(s) is or is increased to maximum, only straight flight
should be maintained while maintaining a bank angle of 3 to 5 degrees
away from the inoperative engine, depending on the airspeed (VYSE
and VMCA resp.).
For turning safely while the asymmetrical thrust is high, gain altitude
first during straight flight to allow for some altitude loss during
reduced thrust turns, because of the increased sideslip (drag) during
turns. The vertical tail is simply not large enough for turns at max.
asymmetrical thrust.It is safer to reduce the thrust a little during the turns to keep the
actual VMCA low. Also consider a long straight-in approach rather than a
tight final turn during which the thrust might have to be increased to
maximum (and control will be lost because actual VMCA
increases above the indicated airspeed).

NOTE

2° to 3° BANK TOWARD OPERATING ENGINE

This note is included in the legend of the Climb Performance Chart - One
Engine Operating in the Piper PA-44 Pilot's Information Manual. It is
included, because not maintaining this bank angle renders the presented
performance data invalid; the airplane might not even be able to maintain
altitude. The bank angle is smaller than 5 degrees, because the presented
performance data requires VYSE, the blue line speed, which is
higher than VMCA. The vertical tail is more effective at higher
speed.
Keeping the wings level or turning means loss of performance; altitude
cannot be maintained on most multi-engine airplanes if this NOTE is
neglected. The reason why this NOTE is included is explained in the papers
presented on the Downloads page.

ONE ENGINE INOPERATIVEAIR MINIMUM CONTROL SPEED 80 KIAS

A similar placard is to be installed in full view of pilots of commuter
Part 23 airplanes to comply with Aviation Regulations (23.1563). The required small bank angle for the listed VMCA to
be valid is regrettably not included on the placard, because this is not
required by the Aviation Regulations, but is essential for flight safety
and performance.

Not maintaining the small bank angle (i.e. straight flight) at
airspeeds as low as VMCA, while the power setting of the
remaining engine is high, is the real cause of most engine failure
related accidents.

It is recommended to require a placard like this one in all Part 23 airplanes.

More options are possible.

VR and V2MIN

The standardized,
AFM-published VMCA is one of the factors for calculating
the rotation speed VR of all multi-engine airplanes, and
for calculating the minimum takeoff safety speed V2MIN
of big Part 25 airplanes. Since this VMCA is
valid only while maintaining a bank angle of 3 to 5 degrees, as to
be specified by the manufacturer, away from the inoperative engine,
both the calculated VR and V2MIN are also
valid only when maintaining the same bank angle (when the thrust
setting is maximum takeoff).
Refer to the
paper for Investigators and Flight Instructors for thorough
explanation of takeoff speeds.

This figure, a
safety improving suggestion of AvioConsult, shows that the
actual VMCA in this example has become higher than VR
because the wings are kept level. Bank angle and rudder
advisories are presented to decrease the actual VMCA to a
safe level to prevent the loss of airplane control. The bank
angle advisory widens up as the airspeed increases.

Also refer to the formal FAA and EASA Flight Test Guides in the reference list on the
Downloads page.

For further details, refer to the paper for investigators and flight instructors
presented on the Downloads page.